1. Field of the Invention
This invention relates to methods for the production of ultra-fine gold particle-immobilized oxides.
Gold has been considered to be typical of metals which are low in chemical activity. It has been reported by some of the present inventors that when gold is dispersed and deposited as ultra-fine particles smaller than 10 nm in diameter on the oxide of such a metal as iron, cobalt, or nickel, the gold becomes a highly active catalyst. ["Chemistry Express", 3, 159-162 (1988), and J. Catalysis", 115, 301-309 (1989)].
2. Prior Art Statement
The following five methods have been developed by some of the present inventors for the production of such an ultra-fine gold particle-immobilized metal oxides as described above.
1) Coprecipitation method (Japanese Patent Public Disclosure SHO 60(1985)-238,148)
A composite having an ultra-fine gold particle immobilized on the oxide of such a metal as iron, cobalt, nickel, or copper is obtained by preparing a mixed aqueous solution having dissolved therein such a water-soluble salt as iron, cobalt, nickel, or copper and a water-soluble compound of gold, neutralizing the mixed aqueous solution with an aqueous alkali solution thereby giving rise to a coprecipitate, washing the coprecipitate with water, and calcining the washed coprecipitate in the air.
2) Uniform deposition and precipitation method (Japanese Patent Public Disclosure SHO 62(1987)-155937)
An ultra-fine gold particle-immobilized oxide is obtained by adding urea and/or acetamide to an aqueous solution of a water-soluble gold compound, immersing a metal oxide carrier in the resultant aqueous solution, heating the aqueous solution thereby inducing hydrolysis of the urea and/or acetamide and gradual formation of ammonia and consequently allowing gold hydroxide to be deposited on the metal oxide in consequence of ensuing increase of the pH value of the aqueous solution, and washing with water and calcining in the air the resultant gold hydroxide-deposited metal oxide.
3) Dropwise neutralization and precipitation method [Japanese Patent Public Disclosure SHO 63(1988)252908]
An ultra-fine gold particle-immobilized oxide is obtained by adding dropwise an aqueous solution of a water-soluble gold compound to an aqueous solution of a metal oxide having a pH value in the range of 7 to 11 while keeping the pH value within the range mentioned above, and washing with water and calcining in the air the resultant gold hydroxide-deposited metal oxide.
4) Reductant addition method [Japanese Patent Public Disclosure SHO 63(1988)252908]
An ultra-fine gold particle-immobilized oxide is obtained by preparing a metal oxide-containing aqueous solution having a water-soluble gold compound dissolved therein and having a pH value in the range of 7 to 11, adding a reducing agent dropwise to the aqueous solution while keeping the pH value in the range mentioned above thereby inducing deposition of gold on the metal oxide, and washing with water and calcining in the air the resultant gold-deposited metal oxide.
5) pH-controlled neutralization and precipitation method [Japanese Patent Public Disclosure SHO 63(1988)-252908]
An ultra-fine gold particle-immobilized oxide is obtained by preparing a metal oxide-containing aqueous solution having a water-soluble gold compound dissolved therein and having a pH value in the range of not less than 11, blowing carbon dioxide gas therein or adding an acidic aqueous solution dropwise thereto thereby adjusting the pH value in the range of 7 to 11 and consequently inducing deposition of gold hydroxide on the metal oxide, and washing with water and calcining in the air the resultant composite metal oxide.
The five methods described above have defects of their own and are subject to restrictions regarding the amount of gold that can be immobilized in an ultra-fine form and the kind of carrier that can be used.
The coprecipitation method of 1), for example, is usable only for the production of a powdery or granular catalyst and not usable for the purpose of dispersing and depositing ultra-fine gold particles on a molded metal oxide or a shaped article having the metal oxide carried thereon. Further, this method has a problem in that the metal oxides on which gold can be effectively carried in the form of an ultra-fine particle are limited to the oxides of Cu, Fe, Co, Ni, etc.
The uniform deposition and precipitation method of 2) has a disadvantage in that accurate control of conditions of the deposition of gold hydroxide is indispensable and the work of this deposition consumes at least several hours's. This method inevitably causes partial precipitation and deposition of gold from the aqueous solution of the gold compound and, therefore, entails a disadvantage in that the utilization efficiency of gold is low and the production cost is high. Moreover, even slight variation in the conditions tends to reduce the uniformity and dispersion of the deposited gold hydroxide and to cause the gold hydroxide to be deposited in large aggregates.
The dropwise neutralization and precipitation method of 3) has a disadvantage in that when the amount of gold loading exceeds 1 wt %, the deposition and precipitation of gold hydroxide on the metal oxide tend to occur unevenly and result in aggregation of ultra-fine gold particles during the course of deposition by calcination.
The reductant addition method of 4) has a disadvantage in that when the specific surface area of the metal oxide carrier is small or when the amount of gold loading is increased, for example, the dissolved gold compound is reduced to form metal particles in solution and is not utilized effectively in the deposition of the ultra-fine gold particles on the support oxide and, consequently, the production cost rises.
The pH control neutralization and precipitation method of 5) has a disadvantage similar to that of the method of 3).
In order to improve the catalytic activities, gas sensitivities, colors of the ultra-fine gold particle-immobilized oxides so as to apply these oxides for use as a catalyst, a gas sensor, or a pigment, it becomes necessary to control the amount of gold loaded and the kind of metal oxides to be used so as to suit the purpose for which the oxides are utilized. In the preparation of ultra-fine gold particle-immobilized titanium oxide as a cosmetic material, for example, the amount of gold to be carried is desired to exceed 10% by weight for the purpose of deepening the tone of its bluish purple color. As the material for a gas sensor which operates at a low temperature and enjoys a long service life, there is required a metal oxide semiconductor having gold carried in an excess amount thereon. By the conventional methods, however, the amount of gold carried cannot be varied over a wide range. Further, these methods cannot be effectively applied to many kinds of metal oxides.
The present inventors continued a study in search of methods for the production of ultra-fine gold particle-immobilized oxides free from the disadvantages suffered by the conventional methods as described above.